The invention relates to chemical removal of alloys containing nickel with gold, phosphorous or chromium from stainless steel or nickel base alloy substrates. More particularly, the invention relates to selective stripping of a gold/nickel, nickel/phosphorous or nickel/chromium alloy from substrates which are formed from 300 to 400 series Stainless Steels or nickel rich, chromium bearing alloys, such as Hastelloy, Inconel or equivalent alloys.
Alloys of gold and nickel, which typically range from between 70% to 80% gold and 18% to 30% nickel are conventionally utilized as a braze to join substrates, such as ferrous and nickel alloys in applications requiring structural strength, yet resistance to extreme heat. Gold/nickel brazing alloys have a melting point in excess of 1000.degree. F. (537.8.degree. C.), and are applied in varying thicknesses by melting a paste or pre-formed segment with flux. In some applications, alloys of nickel (97% to 85%) and phosphorous (3% to 15%) are used, either as brazing alloys or coatings. Nickel/chromium alloys may be applied as a flame spray, or plasma spray, coating. Removal, or "stripping", which is required to disassemble or inspect parts attached or coated with such alloys is preferably accomplished by chemical techniques, rather than by reheating.
While the present invention is not limited to any particular application employing brazing or coating alloys, either of gold/nickel, nickel/phosphorous or nickel/chromium, it is, however, most preferably and advantageously utilized to selectively strip such brazing alloys when used to join segments of internal components of jet engines. For example, gold/nickel brazing alloys are used to secure airfoil vanes in stators used in the turbine section of a jet engine. These are stationary parts, downstream of the combustion chamber, which are exposed to extremely high temperatures and stress. However, mandatory maintenance and inspection procedures require that such components are routinely removed, either for inspection or replacement. While it is necessary to selectively strip the gold/nickel alloy from these components, it must be done in a manner which will not adversely effect, or degrade, the substrate alloys from which the stators or other components are formed. Typically, alloys of iron with chromium and in some instances nickel, and nickel rich, chromium bearing alloys, some of which also contain iron, are used to make such components; for example, Stainless Steel, Hastelloy, Inconel or equivalent alloys.
Furthermore, the time required to complete stripping of the brazing alloy from jet engine components, in particular, is of great importance, in view of the fact that the aircraft may be out of service during such procedures.
Prior art techniques for chemically stripping gold/nickel brazing alloys, for example, involve methods employing a series of steps and various chemical solutions. In order to remove sufficient brazing alloy, a substantial amount of time was required, typically as much as 9 to 18 days. However, with use of ultrasonic agitation, this time could be reduced to 3 to 5 days.
Furthermore, prior art techniques present disadvantages due to employing either substantial amounts of electric current, or use of aqueous solutions containing cyanides, and/or nitro-substituted aromatic compounds and chelating agents, with the resulting toxicity and environmental problems. Finally, prior art methods usually required operation at elevated temperatures, typically ranging from 100.degree. to 185.degree. F. (37.8.degree. to 85.degree. C.).
Exemplary of prior art techniques for removal of brazing alloys containing gold are those set forth in U.S. Pat. No. 3,819,494 and No. 3,958,984. However, the methods provided in accordance with these patents involve the aforementioned disadvantages, particularly with regard to use of cyanides, and are extremely slow in comparison to the present invention.
Another conventionally available gold/nickel braze stripping solution contains concentrated nitric acid, with copper, iron and sodium chlorides. Not only is its action very slow, in comparison with the present invention, but pitting and degradation of the substrate alloy is a problem, particularly with longer exposure times. The substantially longer exposure required for removal of the braze by prior art solutions renders the substrate much more susceptible of attack on the Stainless Steel alloys, with resultant pitting, "smutting" or degradation caused by intergranular attack.
In accordance with the present invention, an improved solution is provided, which, used by itself in preferred embodiments, is fully operable to strip gold/nickel,nickel/phosphorous or nickel/chromium alloys from substrates formed from 300 and 400 series Stainless Steels, Hastelloys, Inconels and equivalent alloys. The novel solution of the invention, and method provided for utilizing the same to strip these brazing or coating alloys, is operable to remove the alloy in substantially less time than is possible with prior art techniques or solutions, and without adversely effecting or degrading the underlying substrate. The novel solution and method of the invention have particular utility for selectively stripping gold/nickel brazing alloys used in jet engines, where minimal time and avoidance of degradation of the underlying substrate is of utmost importance and criticality.
The improved solutions of the invention provide a wide range of operational utility, particularly with respect to temperature of treatment baths, or sprays, which while preferably operable at ambient temperatures, must also be capable of operation at higher temperatures. This is particularly advantageous because it permits use of ultrasonic agitation which tends to raise the temperature, as a result of the conversion of electrical to mechanical energy.